Every IV is just as hard to pass down as every other. Why pass down a 30 when you can have a 31? The only 30 IVs you should try to pass down are for Hidden Power.
Agreed. However, there are certain exceptions. For instance, I hatched this Skorupi yesterday:
31/ 30/ 30/ 1/ 31/ 31
The two 30s were randomly generated.
If I have a physical or special sweeper, I always go for a 31 in speed and the respective attack. 30 doesn't cut it in me. Having that full iv gives me the satisfaction of knowing that I've delivered the maximum amount of damage I can. With some pokemon that don't have any defense (Jolteon, Aerodactyl), you'll only get one chance to make your attack count cause if you don't KO your opponent, you die. The same thing with speed. The quicker one walks away the victor.
On pokes that are relatively loosey-goosey with IV restrictions for speed (such as Weavile, who doesn't need max speed, having less than 31 is ok as long as it can outrun the same pokemon a max speed Weavile can outrun. For example, a 21 IV in speed is acceptable for Weavile (though not particularly good) because it can still hit 373 speed, but a 20 IV in speed won't work, because it at best ties speed with base 120 pokemon.
But the problem with 21 IVs in that example is that you're forced to redirect EVs from other stats to bump up the Speed to that 373 magic number. That's why magic number stats always should have flawless IVs.